The present invention relates to cordless telephones and more particularly to a method for arranging the frequencies in a swap table of a cordless telephone.
The use and demand for cordless telephone systems has greatly increased over the past several years. The expanding market for household digital cordless telephones has necessitated that engineers design more functional and efficient cordless units which meet consumer expectations of clarity and cost efficiency. The large increase in the number of users of these devices has, likewise, required that designers use bandwidth ever more efficiently in order to conserve available bandwidth resources.
A known method of using bandwidth efficiently in digital cordless telephone systems is through the use of frequency spread spectrum technology. This technology, which is also known as frequency hopping, establishes a communication path between a base unit and a hand-held unit of a cordless telephone system over multiple frequencies in non-overlapping units of time. By transmitting and receiving digital voice signals over different frequencies in this manner, any interference over any one particular frequency which may cause poor transmission on that frequency minimally effects the overall transmission when a sufficient number of frequencies are utilized and when frequencies are switched (or hopped) in relatively short intervals of time (e.g.,  less than 10 ms).
The spread spectrum frequency hopping method has been further improved through the use of swap frequencies. The frequencies which are available, but not designated as active hop table frequencies, are relegated to the swap table. However, hardware and cost constraints have generally restricted the number of frequencies available in cordless telephones, for example, even though such devices may be capable of hopping among many more frequencies. During operation of communication devices, when the system finds that a certain hop frequency experiences an inordinate amount of electromagnetic interference, that frequency is xe2x80x9cswappedxe2x80x9d for a pseudo-randomly selected frequency in the swap table. In this way, an attempt is made to minimize interference in the active hop table. According to this known method, however, the frequency selected from the swap table is randomly selected and therefore is unlikely to be the optimal frequency to select from among the frequencies in the swap table. It is possible that the frequency selected from the swap table to replace a degraded hop frequency in the prior art method may have a higher electromagnetic level than the rejected hop frequency itself.
Accordingly an apparatus and method are needed which optimally arrange and select a swap frequency so that electromagnetic energy in the selected channel is at a minimum so that a hop frequency experiencing interference can be replaced and the communication improved.
In one aspect of the present invention, a method for arranging a set of frequencies assigned to a swap table in a cordless telephone comprises the steps of (i) identifying an electromagnetic energy level at each frequency in the set of swap frequencies; and (2) sorting the swap table frequencies in order of each frequency""s respective electromagnetic energy level.
In another aspect of the invention another set of frequencies comprising a hop table is used in conjunction with the swap table frequencies such that when a hop frequency fails to meet a predetermined criterion: (1) that hop frequency is replaced by the swap frequency in the first position in the swap table and (2) the degraded hop frequency from the hop table is placed in the last position of the swap table.
In another aspect of the invention, a system for arranging the frequencies in a swap table of a cordless telephone according to each frequency""s electromagnetic energy level is disclosed which comprises: (1) radio frequency reception circuitry which receives the electromagnetic energy of the swap frequencies; (2) a controller which determines the magnitude of the electromagnetic energy content of each of the frequencies assigned to the swap table (3) a memory storage which stores the swap table; and (4) a controller which arranges the frequencies in the swap table in order of the electromagnetic energy content of each of the swap frequencies.